It is possible to realize high densities in various technical fields, respectively, by reducing a diameter of so-called a light spot, which is a focal point onto which light is converged. For example, in the field of optical recording, data is recorded and reproduced on and from a recording medium, respectively, with the use of a laser beam. A reduction in diameter of a light spot enables high-density recording/reproducing. Further, in the field of optical fabrication, a material such as a resin or glass is fabricated with the use of a laser beam. A reduction in diameter of a light spot enables a finer fabrication of such a material. Furthermore, in the field of measurement using a microscope or the like, a reduction in diameter of a light spot allows an increase in measurement resolution.
In view of the circumstances, there have been demands for a reduction in diameter of a light spot in each of the technical fields, utilizing light, such as the optical recording, optical fabrication and the measurement by means of a microscope. However, in a case where normal light is used, a size of a light spot is limited to that almost equal to a wavelength of light due to a diffraction limit, and so it was difficult to further reduce the size of the light spot. Therefore, a method utilizing near-field light which exists locally is attracting attention as a method for forming a light spot smaller than a diffraction limit of light in spite of using normal light.
The near-field light is localized light (electromagnetic field) which is generated while a minute structure smaller than the wavelength of light (e.g. aperture) is irradiated by the light. The near-field light exists only in the vicinity of the aperture. The near-field light generated in the vicinity of the aperture remains in the vicinity of the aperture and is not propagated to other area.
In a case where light from a light source is incident on an aperture whose diameter is larger than the wavelength of the light, the light is partially blocked, but propagates and passes through the aperture. As such, no near-field light is generated. In contrast, in a case where an aperture has a diameter smaller than the wavelength of incident light, the incident light hardly passes through the aperture, and near-field light is generated in the vicinity of the aperture. The near-field light thus generated has an intensity distribution whose size is substantially the same as the diameter of the aperture. This makes it possible to obtain a light spot smaller than the diffraction limit in the vicinity of the aperture.
The light spot thus obtained whose diameter is reduced can be suitably used in a laser-assisted magnetic recording method. The laser-assisted magnetic recording method has been attracting attention as a promising technique for a next-generation high-density magnetic recording. The laser-assisted magnetic recording method is a method for conducting a magnetic recording with respect to a magnetic recording medium having high thermal fluctuation resistance and high coercive force. Specifically, light is converged onto a surface of the magnetic recording medium so that the magnetic recording medium has a local temperature rise. This causes a reduction in coercive force of the magnetic recording medium. This allows a normal magnetic head to carry out a magnetic recording with respect to the magnetic recording medium.
However, a light spot obtained with the use of the above method has low light use efficiency. This is because the light which can not be converged to a light spot having a diameter of not more than a wavelength of the light is incident on the aperture which is not more than the wavelength of the light. That is, when a light source has the same intensity as a conventional one, an intensity of the near-field light becomes has intensity smaller by an amount corresponding to the size of the aperture than an amount corresponding to the size of the light incident on the aperture. Further, the intensity of the light sharply declines as the light is farther away from a point where the light is generated because the light is localized.
Therefore, the following method is used. Specifically, according to this method, (i) light is caused to be incident on an aperture made from a metal film so that a surface plasmon polariton is generated on the metal film, and then (ii) the surface plasmon polariton is amplified so that strong near-field light is generated. Further, the near-field light localized in the aperture can be propagated toward any position because a surface plasmon polariton is used.
Patent Literature 1 discloses a technique in which near-field light, which is propagated toward any position by a surface plasmon polariton, is used in the laser-assisted magnetic recording method. The following description deals with the technique disclosed in the Patent Literature 1 with reference to FIGS. 14 and 15. FIG. 14 is a perspective view illustrating a conventional read/write head 101 used in the laser-assisted magnetic recording method. FIG. 15 is a cross-sectional view of the read/write head 101, which cross-sectional view is obtained when the read/write head 101 of FIG. 14 is viewed from its side.
According to the read/write head 101 disclosed in the Patent Literature 1, an irradiated surface 104 is irradiated by a laser beam 102 from a top surface of a near-field light head 103; and an electric vibration wave (surface plasmon polariton) is excited in a laser spot 105 formed on the irradiated surface 104. As shown in FIG. 15, the electric vibration wave thus excited in the laser spot 105 is propagated toward a radiation section 107 via a waveguide 106. A recording medium 110 is irradiated by near-field light 108 via the radiation section 107. While the recording medium 110 is thus irradiated by the near-field light 108 from the radiation section 107, the recording medium 110 has a heated portion. Under the circumstances, a magnetic head 109 records information on the heated portion.
In the near-field light head 103, (i) a surface which is irradiated by the laser beam 102 and (ii) a surface on which the radiation section 107 is provided are successively connected to each other so that the electric vibration wave changes its propagation direction. In the near-field light head 103, the irradiated surface 104 becomes narrower so that the electric vibration wave excited in the laser spot 105 is concentrated in the radiation section 107. However, in this method, it is impossible to change a propagation direction of a surface plasmon polariton within a surface in which the surface plasmon polariton is propagated. This causes less flexibility of the provision of a recording magnetic field generating section and a reading element.
In view of the circumstances, Patent Literature 2 discloses a technique for changing a propagation direction of a surface plasmon polariton within a surface in which the surface plasmon polariton is propagated. The following description deals with the technique disclosed in the Patent Literature 2 with reference to FIGS. 16 and 17. FIG. 16 is a perspective view schematically illustrating an arrangement of a conventional metal film 201, having two kinds of thickness, for causing a surface plasmon polariton to be propagated and refracted, the metal film 201. FIG. 17 is a perspective view showing a surface plasmon lens 211 for converging a surface plasmon polariton.
As shown in FIG. 16, the metal film 201 includes a first metal film 202 and a second metal film 203 which are different in thickness. The difference in thickness gives rise to a difference in effective refraction index. This causes a surface plasmon polariton 204 excited in the first metal film 202 to be refracted at a boundary between the first metal film 202 and the second metal film 203.
With the arrangement, it is possible to change the propagation direction of the surface plasmon polariton 204 at the boundary between the first metal film 202 and the second metal film 203 which are difference in thickness. It is thus possible, with such a simple arrangement, to propagate a surface plasmon polariton toward any position.
Further, as shown in FIG. 17, the surface plasmon lens 211 is arranged such that a metal film 215 is provided on a top surface of a dielectric layer 216; and (i) a first dielectric layer 213 having a low refractive index and (ii) a second dielectric layer having a refractive index higher than the first dielectric layer 213 are provided on a surface of the metal film 215 which is opposite to a surface in contact with the dielectric layer 216. The first dielectric layer 213 can be air. Therefore, the first dielectric layer 213 is not shown in FIG. 17.
In the surface plasmon lens 211, when a laser beam 212 is incident between (i) the metal film 215 on which the first dielectric layer 213 is provided and (ii) dielectric layer 216, the surface plasmon polariton 204 is excited between the metal film 215 and the first dielectric layer 213. The effective refractive index of the metal layer 215 changes depending on a medium with which the metal layer 215 is in contact. Therefore, the surface plasmon polariton 204 propagates toward the second dielectric layer 214, and is refracted at the boundary between the first dielectric layer 213 and the second dielectric layer 214.
With the arrangement, it is possible to change the propagation direction of the surface plasmon polariton 204 at the boundary between the first dielectric layer 213 and the second dielectric layer 214, which are provided on the metal film 215 and are different in refractive index. It is thus possible, with such a simple arrangement, to propagate a surface plasmon polariton toward any position.
Note however that the effective refractive index of the surface plasmon lens 211 disclosed in the Patent Literature 2 varies depending on not only the refractive indices of the first dielectric layer 213 and the second dielectric layer 214 but also the thickness of the metal film 215. Further, in the surface plasmon lens 211, the surface plasmon polariton 204 is excited in an asymmetric mode (later described). Therefore, as described in a Non-patent Literature 1, the thinner the metal film 215 becomes, the larger the effective refractive index becomes and the shorter the propagation length becomes.
As such, if the thickness of the metal film 215 is reduced so that the surface plasmon polariton 204 is sufficiently refracted at the boundary between the first dielectric layer 213 and the second dielectric layer 214, then the propagation length will become short in the surface plasmon lens 211, i.e., several times the wavelength of the surface plasmon polariton 204 or not more than the wavelength.
Further, according to the surface plasmon lens 211, it is only possible to obtain near-field light whose intensity is reduced by an amount corresponding to the thickness of the second dielectric layer 214, in a case of utilizing the near-field light which is excited in a direction perpendicular to the metal film 215 on which the first dielectric layer 213 and the second dielectric layer 214 are provided.
Here, the effective refractive index n of a metal film is expressed as follows:n=Re(βc/ω)
where β indicates a wave number vector in a propagation direction of a surface plasmon polariton, c indicates the speed of light, and ω indicates the angular frequency of the surface plasmon polariton.
Further, the intensity of a surface plasmon polariton is attenuated as the surface plasmon polariton propagates on a metal film. A distance at which the intensity of a surface plasmon polariton becomes 1/e as large as the original one is referred to as a propagation length. The propagation length L which is a parameter indicating intensity attenuation of a surface plasmon polariton is expressed as follows:L=1/Im(β)/2
The wave number vector β in a propagation direction of a surface plasmon polariton varies depending on the frequency of the surface plasmon polariton, a mode of the surface plasmon polariton, a metal material constituting a metal film, the thickness of the metal film, and a material with which the metal film is in contact.
The following description deals with modes of a surface plasmon polariton. Generally, a surface plasmon polariton which is propagated on a metal film has two modes. One of them is a symmetric mode in which surface plasmon polaritons on both sides of a metal film are symmetrically coupled. Such surface plasmon polaritons are excited in Kretchmann configuration which is later described. The other of the two modes is an asymmetric mode in which surface plasmon polaritons on both sides of a metal film are asymmetrically coupled. Such surface plasmon polaritons are excited in Otto configuration which is later described.
Therefore, the wave number vector β in a propagation direction of a surface plasmon polariton has two values corresponding to the two modes of the surface plasmon polariton. The Non Patent Literature 1 deals with this in detail.